Rip current

A rip current is a powerful, localized, and narrow stream of water. It moves directly away from the shoreline, cutting through the lines of breaking waves. You can visualize a rip current as a river flowing out to sea.

While they are not the same as an undertow, they are often called "rip tides" by mistake. It is important to note that a rip current does not pull a person under the water. Instead, it carries floating objects and people away from the shore toward the open ocean. Because they can move very quickly, they are a major safety concern at beaches. In the United States, rip currents caused an average of 71 deaths by drowning every year between 2013 and 2022.

To understand how a rip forms, we must look at how waves interact with the shore. Wind and breaking waves push surface water toward the land. This action causes a slight rise in the water level along the coast. This excess water seeks the route of least resistance to return to the open ocean. If there is a deeper area, such as a gap in an offshore sandbar or a reef, the water will rush through that opening. This process creates the rip current. The water moves toward the gap via paths called feeder currents. Once it reaches the gap, it forms a tight, rapid flow known as the "neck."

After the water passes the breaking waves, it spreads out and loses power in an area called the "head."

Scientists use specific concepts to explain this movement, such as radiation stress. Radiation stress is the force or momentum flux exerted on the water by a wave. As a wave enters shallow water and shoals, it increases in height before it breaks. This height increase raises the radiation stress because of the weight of the upward-pushed water. This causes a local drop in the water level called a "setdown." When the wave breaks and shrinks, the radiation stress decreases. This leads to an increase in the water level known as a "setup." When waves break over a sandbar with a gap, the setup occurs on the sandbar while the setdown continues over the gap. This difference in water levels forces a strong outward flow through the gap.

Rip currents can appear in many different environments. They occur along the coasts of oceans, seas, and even large lakes. They require waves with enough energy to drive the process. These currents often form on shores that slope gradually. They also appear where underwater topography, like sandbars or reefs, encourages water to flow out in specific spots. The location of a rip can be hard to predict. Some rips recur in the same place, but others appear and disappear suddenly. This depends on the direction of the swells and the shape of the ocean floor. Factors like piers, jetties, or even crossing wave trains can cause the variability in wave breaking that leads to a rip.

Researchers have studied the physics of these currents in great detail. At the Scripps Institute of Oceanography, scientists studied the vorticity and inertia of rip currents. Vorticity refers to the rotation or spinning motion within the water. Their models showed that as a fast rip current moves away from shallow water, its vorticity increases while its width decreases. Other data from Scripps using Sector-Scanning Doppler Sonar provided specific measurements. In La Jolla, California, rip currents were found to last several minutes. They were observed to reoccur between one and four times per hour. These currents also created a specific shape, described as a wedge with a 45-degree arch.

Identifying a rip current from the shore can help swimmers stay safe. A rip often looks like a break in the pattern of the waves. The water in the rip may look relatively flat compared to the white, breaking waves nearby. You might also see a "river" of foam being carried out to sea.

The color of the water can also change. A rip might look more opaque, cloudy, or muddy than the surrounding ocean. Depending on the sun, it might appear darker or lighter. While most people struggle to identify these signs, experienced lifeguards use them to monitor beach safety. In the United States, organizations like NOAA provide signs to help people recognize these dangers.

If you are caught in a rip current, the most important thing is to stay calm. A common mistake is trying to swim directly against the current to get back to shore. This can lead to exhaustion and panic. Since the current is often faster than a human can swim, you should not fight it. Instead, think of the rip as a moving treadmill. You should swim at a right angle to the flow, which means swimming parallel to the beach. Once you are out of the narrow current, you can swim back to land at an angle. If you cannot swim out, you can float or tread water until the current dissipates or help arrives.